Internal combustion engines, and diesel engines in particular, are known to emit oxides of nitrogen under various operating conditions. Emissions limits promulgated by the US Federal government are stringent and are projected to become even more so. Accordingly, it is of great interest to engine and vehicle manufacturers to develop strategies for continuous removal of NOx from the exhaust streams of engines and to match those emission control strategies to the range of combustion modes possible in such engines.
One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such operation is possible using compression-ignition (diesel) and lean-burn spark-ignition engines. When an engine operates with a lean (excess oxygen) air/fuel ratio, the resultant combustion temperature is lower, leading to decreased engine-out NOx emissions; however, commercial application of lean-operating engines is limited due to lack of effective methods to remove NOx under a lean exhaust condition. Thus, efficient reduction of nitrogen oxides (NOx=NO+NO2) from diesel and lean-burn gasoline exhaust is important to meet future emission standards and improve vehicle fuel economy.
Reduction of NOx emissions from an exhaust feedstream containing excess oxygen is a challenge for vehicle manufacturers. By way of example, it has been estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOx conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOx levels. For practical applications, the conversion efficiency must be obtained at a low temperature operating range (e.g., 200-350° C.) occurring during the aforementioned FTP cycle and at a higher temperature operating range (e.g., 450-550° C.) occurring during a high speed test cycle (e.g., US06 Federal Test Procedure).
Several potential aftertreatment systems have been proposed for vehicle applications. One approach comprises using an aftertreatment system including injecting a NOx reductant, e.g., urea, upstream of an urea-SCR catalyst to form ammonia, thereby to reduce NOx to N2 and water. Use of urea as a reductant necessitates a urea storage and distribution infrastructure and an on-vehicle monitoring system for this secondary fluid, and may have potential problems in cold weather climates due to the relatively high freezing point (−12° C.) of the urea solution and deposits that can form in the exhaust at low temperatures.
NOx storage catalysts typically require large catalyst volumes, large amounts of PGM or other precious metals, and low sulfur fuel for efficient storage operation. Such systems also require periodic catalyst regeneration involving fuel injection to generate high exhaust gas temperatures and injection of reductants such as H2 or hydrocarbon fuel to regenerate the storage material of the catalyst.
Selective Catalytic Reduction (SCR) of NOx using hydrocarbons (HC-SCR) has been studied extensively as a potential alternative method for the removal of NOx under oxygen-rich conditions. Ion-exchanged base metal zeolite catalysts (e.g., Cu-ZSM5) have typically not been sufficiently active under typical vehicle operating conditions, and are susceptible to degradation by sulfur dioxide and water exposure. Catalysts employing platinum-group metals (e.g., Pt/Al2O3) operate effectively over only a narrow temperature window and are highly selective towards N2O production.
Making NH3 in a rich or reducing exhaust is relatively easy because NH3 is a reducing species. However, as noted above, making NH3 in lean or oxidizing exhaust is difficult but possible with an appropriate catalyst. NH3 usually reacts with oxygen but a few catalysts will allow ammonia and other N-containing species to survive. Catalytic devices using alumina-supported silver (Ag/Al2O3) have received attention because of their ability to selectively reduce NOx under lean exhaust conditions with a wide variety of hydrocarbon species. (Since the price of silver historically has been less than 1/100 that of platinum, silver is not considered to be a precious metal in our discussion here.)
The use of partially-oxidized hydrocarbons (e.g., alcohols) over Ag/Al2O3 allows reduction of NOx at lower temperatures. However, such reductants typically are unavailable on-board a vehicle. In other prior art approaches HC-SCR over Ag/Al2O3 catalysts utilize light hydrocarbons (e.g., propene, propane) and heavier fuel-component hydrocarbons (e.g., octane, decane) as a reductant. NOx reduction using lighter hydrocarbons already present as the combustion products in engine exhaust yields conversion at higher temperatures, but for Ag/Al2O3 catalysts to be considered as candidates for practical use, the NOx reduction must be shifted to a lower temperature region and the fuel on-board the vehicle must be used as the reductant.
U.S. Pat. Nos. 6,057,259 and 6,284,211 disclose high percentage conversion of NO to N2 over silver catalyst with ethanol as the reductant. Ammonia and other N-containing species are disclosed as exiting the silver catalyst and may be removed by a second catalyst including an NH3-SCR catalyst.
World Patent No. WO 2006/093802 and Published US Patent Application No. 2006/0228283 disclose combining a Ag HC-SCR catalyst with a range of other catalysts, including NH3-SCR catalysts, to give higher NOx conversion than either catalyst separately. Ammonia is cited as one species exiting the Ag catalyst.
Published US Patent Application No. 2007/0059223 discloses the combination of a Ag HC-SCR catalyst with another HC-SCR or partial oxidation catalyst to achieve high NOx reduction efficiency. The Ag catalyst uses ceria as an additive and the disclosure explicitly includes HC injection in the system.
Published US Patent Application No. 2008/0066456 A1, the relevant portions of which are incorporated herein by reference, discloses control of NOx emissions from a silver catalyst by controlling the HC/NOx ratio and level of H2. There is no disclosure of a second NH3-SCR catalyst or ammonia.
The prior art does not disclose to deliberately increase the amount of ammonia produced in a first catalyst, nor to control the amount of ammonia and other N-containing species formed with HC and/or H2 in a first catalyst, nor to combine such control with an ammonia/NO2/NO sensor, as is partially the subject of the present invention.
What is needed in the art is an inexpensive and effective method and apparatus to selectively reduce NOx in an exhaust gas feedstream for vehicles and other applications of lean-burn internal combustion engines.
It is a principal object of the present invention to reduce the size, complexity, and cost of a continuously-operable high-efficiency NOx abatement system using only non-platinum group metals.